In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
A crucial step in deposition of different Al2O3 polymorphs is the nucleation step. κ-Al2O3 can be grown in a controlled way on {111} planes of TiN, Ti(C,N) or TiC having the face-centered cubic structure. Transmission electron microscopy (TEM) has confirmed the growth mode which is that of the close-packed (001) planes of κ-Al2O3 on the close-packed {111} planes of the cubic phase with the following epitaxial orientation relationships: (001)κ//(111)TiX; [100]κ; [112]TiX. An explanation and a model for the CVD growth of metastable κ-Al2O3 has been proposed (Y. Yoursdshahyan, C. Ruberto, M. Halvarsson, V. Langer, S. Ruppi, U. Rolander and B. I. Lundqvist, Theoretical Structure Determination of a Complex Material: κ-Al2O3, J. Am. Ceram. Soc. 82(6)1365-1380 (1999)).
When properly nucleated, κ-Al2O3 layers can be grown to a considerable thickness (>10 μm). The growth of even thicker layers of κ-Al2O3 can be ensured through re-nucleation on thin layers of, for example, TiN inserted in the growing κ-Al2O3 layer. When nucleation is ensured the κ→α transformation can be avoided during deposition by using a relatively low deposition temperature (<1000° C.). During metal cutting high temperature conditions may be created such that the κ→α phase transformation has been confirmed to occur. In addition to phase stability there are several reasons why α-Al2O3 should be preferred for many metal cutting applications. α-Al2O3 exhibits better wear properties in cast iron, as discussed in U.S. Pat. No. 5,137,774. Further, a layer which has been nucleated as α-Al2O3 does not contain any transformation cracks and stresses. Nucleated α-Al2O3 should be more ductile than α-Al2O3 formed totally or partially as a result of phase transformation, and even more ductile than κ-Al2O3, the plasticity of which is limited by the defect structure.
However, a stable α-Al2O3 phase has been found to be more difficult to be nucleated and grown at reasonable CVD temperatures than the metastable κ-Al2O3. It has been experimentally confirmed that α-Al2O3 can be nucleated, for example, on Ti2O3 surfaces, bonding layers of (Ti,Al)(C,O), as shown in U.S. Pat. No. 5,137,774, or by controlling the oxidation potential using CO/CO2 mixtures, as shown in U.S. Pat. No. 5,654,035. The bottom line in all these approaches is that nucleation must not take place on the 111-surfaces of TiC, TiN, Ti(C,N) or Ti(C,O,N), otherwise κ-Al2O3 is obtained.
It should also be noted that in conventional prior art methods higher deposition temperatures are usually used to deposit α-Al2O3. When the nucleation control is not complete, as is the case in many conventional products, the resulting α-Al2O3 layers have, at least partly, been formed as a result of the κ-Al2O3→α-Al2O3 phase transformation. This is especially the case when thick Al2O3 layers are considered. These kind of α-Al2O3 layers are composed of larger grains with phase-transformation cracks. These coatings exhibit much lower mechanical strength and ductility than α-Al2O3 coatings that are composed of nucleated α-Al2O3.
The control of the α-Al2O3 polymorph on an industrial scale was achieved in the beginning of the 1990's with commercial products based on U.S. Pat. No. 5,137,774. In addition, α-Al2O3 has been deposited with preferred coating textures. In U.S. Pat. No. 5,654,035 an alumina layer textured in the (012) direction, and in U.S. Pat. No. 5,980,988 an alumina layer textured in the (110) direction are disclosed. In U.S. Pat. No. 5,863,640 a preferred growth either along (012), (104), or (110) directions is disclosed. U.S. Pat. No. 6,333,103 describes a modified method to control the nucleation and growth of α-Al2O3 along the (10(10)) direction.